1. Field of the Invention
The present invention relates to a developing apparatus which develops an electrostatic latent image held on an image carrier such as a photosensitive material or a dielectric by a mono-component developer.
2. Description of the Related Art
In electronic photocopying machines, electronic photoprinters, and other electrostatic recording apparatuses, an electrostatic latent image is drawn on an image carrier such as a photosensitive material or a dielectric, that electrostatic latent image is developed electrostatically as a charged toner image by a developer, then the charged toner image is electrostatically transferred to a recording medium such as recording paper, then is fixed on the recording medium by heat, pressure, light, etc.
As a developer used in the development process, in general two-component developers comprised of a toner component (fine particles of a coloring resin) and a magnetic component (fine magnetic carrier) are widely known. A developing apparatus using a two-component developer is provided with a developer holding container, an agitator for agitating the two-component developer in the developer holding container and causing frictional charging between the toner particles and the magnetic carrier, and a magnetic roller for attracting part of the magnetic carrier by magnetic force and forming a magnetic brush, that is, a development roller. Part of the development roller is made to be exposed from the developer holding container and face the image carrier. Toner particles electrostatically deposit on the magnetic brush formed on the circumference of the development roller. By the rotation of the development roller, the toner particles are transported to the region facing the image carrier along with the magnetic brush, that is, the development region, where the electrostatic latent image is developed. In short, the magnetic carrier in the two-component developer is given two functions: the function of causing charging friction of the toner and the function of transporting the toner to the development region.
In such a developing apparatus for a two-component developer, there is the advantage that the transportability of the toner particles, which governs the quality of the developed toner image, that is, the quality of the recorded toner image, is relatively excellent, but to maintain that excellent toner transportability, the ratio of the components of the toner particles and the magnetic carrier has to be maintained within a predetermined range and the magnetic carrier has to be periodically replaced. That is, the toner component is consumed by the development, so the toner component must be suitably resupplied. Also, the magnetic carrier must be replaced when degraded.
Therefore, attention has been focused on a developing apparatus using a mono-component developer comprised of only fine particles of a coloring resin, that is, the toner component, as a developing apparatus which does not require the troublesome maintenance as in the case of a two-component developer. In the case of a mono-component developer, especially a nonmagnetic type mono-component developer, however, how the toner particles are charged and how they are transported to the development region become important issues. This is to say that the quality of the developed toner image, that is, the quality of the recorded toner image, is largely governed by the charging characteristic of the toner component and the transportability of the toner component.
In a conventional developing apparatus using a mono-component developer, as the transporter of the developer for transporting the toner to the development region, use is made of an elastic development roller formed from an electroconductive synthetic rubber material, an electroconductive porous synthetic rubber material, etc. The elastic development roller is placed inside the toner holding container and part of it is exposed from the toner holding container and placed in contact with the image carrier. When the elastic development roller is made to rotate, toner particles deposit on the rotating circumferential surface by the frictional force, whereby a toner layer is formed. The toner particles are transported to the development region by this. To develop the electrostatic latent image with a uniform development density, however, the thickness of the toner layer must be kept uniform.
Therefore, use has been made of a blade, roller, or other thickness-regulating member for the elastic development roller. This removes the excess toner from the toner layer and helps make the toner layer uniform. On the other hand, regarding the charging of the toner, use is made of the frictional static electricity on the elastic development roller or thickness-regulating member, but this frictional static electricity is easily affected by changes in the environment, such as the temperature and humidity, so one practice is to form the thickness-regulating member from a conductive material and apply a voltage of a predetermined polarity so as to positively implant a charge to the individual toner particles at the time of regulating the thickness of the toner layer. Of course, when the frictional static electricity is used, the material of the toner component, the material of the elastic development roller, and the material of the thickness-regulating member are selected so as to give a predetermined charge of the desired polarity to the toner particles. Further, when charge implantation is used, the material of the thickness-regulating member is limited to an electroconductive material.
A problem with the developing apparatus for a mono-component developer such as the one explained above, however, it has been pointed out, is that it is difficult to maintain the uniformity of the thickness of the toner layer by the thickness-regulating member stably over a long period. For example, it has been proposed to use, as the thickness-regulating member able to perform charge implantation, for example, a metal rigid blade having a sharp edge and to engage the edge portion elastically with the elastic development roller to remove the excess toner particles and thereby make the toner layer uniform in thickness. In this case, to ensure the uniformity of the thickness of the toner layer, it is necessary to make the processing precision of the sharp edge portion of the metal rigid blade 2 .mu.m or less. That is, this is because the size of individual toner particles is generally from about 5 to about 10 .mu.m, so if the processing precision of the edge portion is more than 2 .mu.m, uneven streaks will be left on the surface of the toner layer. These streaks will appear as white streaks or black streaks in the recorded toner image. For example, even if it were possible to make the processing precision of the sharp edge portion of the metal rigid blade 2 .mu.m or less, such an edge portion would be susceptible to damage and also the processing cost would become extremely high, so it would be extremely difficult to commercialize this.
It has also been proposed to bring the flat surface of a metal rigid blade or the rotational surface of a metal roller into press-contact with the elastic development roller and regulate the pressure on the toner layer. In this case, it is possible to process the flat surface or the rotational surface at a relatively low cost and a high precision, but the pressure of the metal blade or metal roller applied to the elastic development roller to regulate the thickness of the toner layer to a predetermined thickness must be made considerably large. Therefore, the toner particles are crushed and can physically become fixed to the flat surface or the rotational surface. Of course, even if the toner particles become fixed on the flat surface of the metal rigid plate or the rotational surface of the metal roller, uneven streaks will remain on the surface of the toner layer and those streaks will appear in the recorded toner image in the same way as in the above case. Note that when a hard polymer material etc. is used as the material of the thickness-regulating member, it is not possible to control the charging of the toner particles by charge implantation.
It has been also proposed to use a leaf spring member as a metal thickness-regulating member which is able to stably regulate the thickness of the toner layer over a long period and which can be processed at a relatively low cost and a high precision. This leaf spring member is chamfered at its front edge to give it roundness. The rounded front edge is elastically pressed against the elastic development roller by the spring force of the leaf spring member itself. By this, the thickness of the toner layer is regulated. When such a leaf spring member is used as the toner layer thickness-regulating member, the majority of the excess toner is removed from the toner layer formed on the circumference of the elastic development roller by the rounded front edge of the leaf spring member, then the flat surface of the leaf spring member is used to regulate the thickness of the toner layer, so the pressing force of the flat surface on the elastic development roller can be made relatively small and thus it is possible to prevent the toner particles from fixing on the flat surface. Further, the high precision processing of the flat surface of the leaf spring member and the high precision processing of the rounded front edge of the same can be performed at a relatively low cost. In addition, the rounded front edge is far less susceptible to damage compared with the edge portion of the metal rigid blade mentioned above.
A problem with the leaf spring member explained above, however, it has been pointed out, is the ease of vibration of the leaf spring member L and therefore cyclic fluctuation of the thickness of the toner layer at the time of regulation of the thickness of the toner layer. Of course, if the leaf spring member vibrates and the thickness of the toner layer fluctuates, not only will the development density of the electrostatic latent image be affected, but also at areas where the toner layer has become thicker, the charge of the toner particles will become insufficient and thus there will be contamination of the background region of the electrostatic latent image by the toner particles, i.e., so-called fogging.
On the other hand, even with a leaf spring member, when regulating the thickness of the toner layer and charging the toner particles by charge implantation, the thickness must be made equal to the diameter of the toner particles. In other words, the toner layer should be formed as a single layer of the toner particles. This is because when the thickness of the toner layer is greater than the diameter of the toner particles, the toner layer will include toner particles not able to directly contact the leaf spring member. Such toner particles will not be sufficiently implanted with charges and the amount of charge will become insufficient. Of course, toner particles with insufficient charging become a factor causing fogging.